Theory of wind accretion

TL;DR

This paper reviews wind accretion regimes in high-mass X-ray binaries, focusing on the physics of quasi-spherical accretion onto neutron stars, including the supersonic and subsonic regimes, and relates theory to observations of specific pulsars.

Contribution

It provides a detailed theoretical framework for understanding the subsonic and supersonic accretion regimes and connects these models with observational data from X-ray pulsars.

Findings

Identification of the critical luminosity separating accretion regimes.

Calculation of plasma entry rates and angular momentum transfer.

Estimation of magnetospheric boundary layer width for specific pulsars.

Abstract

A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about $4\times10^{36}$ erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.